Prince George’s County police are searching for a missing ­department-issued rifle after an officer reported that he failed to put the weapon back into his cruiser when retrieving a spare tire he lent to a colleague.

The semiautomatic weapon was gone for almost five hours before the officer realized it was missing and a search for it began, county police said after an inquiry from The Washington Post.

Deputy chief Hank Stawinski said tracking down the weapon, which was unloaded, is “priority number one” for the department.

“This is an important piece of equipment,” Stawinski said. “It is something we’re very concerned about losing, and we’re very concerned about getting it back into our hands as quickly as possible.”

The rifle went missing shortly after 6 p.m. Thursday when a patrol officer pulled over in the 7000 block of Livingston Road in the Oxon Hill area to retrieve a spare tire from a fellow officer who earlier needed to repair two flats, police said.

After unloading items from the cruiser to put the spare back into the trunk, the officer placed the AR-15-style rifle somewhere near the cruiser, police said. As the officer reloaded his trunk, he grabbed everything but the rifle.

The officer realized at about 11 p.m. Thursday that the rifle was missing. After a brief search of his home and cruiser, the officer reported the missing weapon to his supervisor, police said. The department then launched a search for the rifle.

Detectives believe that someone may have picked up the weapon after the officer left.

“It does appear to be a deeply unfortunate mistake,” Stawinski said. “At this point, every possible avenue for recovering this rifle is being investigated.”

Police had earlier reported that the officer lost the rifle while changing two tires and that the rifle had been missing for nearly 12 hours before it was reported. After further investigation, they updated details of the incident Saturday evening.

Depending on make and model, AR-15-style rifles are capable of firing several rounds a second.

Only specially trained officers are issued one of the department’s roughly 300 assault weapons, Stawinski said. The department assigns the rifles to certain patrol officers in preparation for possible active-shooter incidents, when it may take longer for someone on the SWAT team to respond.

Department policy requires weapons to be in an officer’s physical control at all times. The department is reviewing the incident to determine what disciplinary action the officer may face.

Anyone with information about the missing weapon should contact police, Stawinski said.

“To the best of my recollection,” Stawinski said, “we’ve never had a weapon go missing like this.”